A thin film transistor including a semiconductor film formed over an insulating surface is an essential semiconductor element for a semiconductor device. Since manufacturing the thin film transistor is limited in terms of allowable temperature limit of a substrate, a thin film transistor including, as an active layer, amorphous silicon which can be formed at a relatively low temperature, polysilicon which can be obtained by crystallization by using a laser beam or a catalytic element, or the like is mainly used as a transistor for the semiconductor display device.
In recent years, a metal oxide having semiconductor characteristics which is referred to as an oxide semiconductor has attracted attention as a novel semiconductor material which has both high mobility, which is a characteristic of polysilicon, and uniform element characteristics, which is a characteristic of amorphous silicon. The metal oxide is used for various applications. For example, indium oxide is a well-known metal oxide and used as a material of a transparent electrode included in a liquid crystal display device or the like. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor including such a metal oxide having semiconductor characteristics in a channel formation region has been known (Patent Documents 1 and 2).